An Athymic Mouse Research Facility has been established at the University of California, San Diego. The mouse colony contains close to 5,000 animals. Approximately one half of them are used in experiments with transplantable human tumors. Twenty-seven human tumor lines are being maintained in the mice and in tissue cultures. This number is constantly increasing. Hormonal requirements of human tumors for growth in serum-free culture are under study. The absence of serum in culture enables one to discover hitherto unsuspected hormonal dependencies of these tumors. Clinical chemotherapy results on humans are correlated with those in mice or cell cultures. In these studies new and modified therapeutic agents (e.g., interferon and polymeric drugs) potentially useful in humans are being used. The effects of nutrition and growth requirements on human tumors are also being explored.